In general, in a production process of a substrate, such as a wafer, a substrate cleaning process using dipping, single-wafer, or like method is performed to remove factors responsible for defect of semiconductor devices. Such factors include organic substances, metal impurities, particles (fine particles), and chemical oxide films.
A substrate cleaning process employs various types of cleaning methods depending on the purpose. In particular, when the dipping method is employed to remove particles, a substrate is dipped into a cleaning liquid contained in a cleaning bath, and ultrasonic waves are irradiated onto the cleaning liquid where the substrate is dipped. Such ultrasonic waves are called megasonic waves, whose frequency range is around 1 MHz. It is generally recognized that use of ultrasonic waves in a frequency range of around 1 MHz reduces damage to the substrate and increases the effectiveness of cleaning micro particles of submicron sizes on the substrate.
When megasonic waves are irradiated onto ultrapure water to remove particles from the substrate, the particle removal efficiency is known to be affected by the dissolved nitrogen concentration of a cleaning liquid. Specifically, in a specific range of the dissolved nitrogen concentration of a cleaning liquid, the particle removal efficiency for substrate increases. If the dissolved nitrogen concentration of a cleaning liquid is monitored and controlled to be in a certain range during the cleaning process, effective removal of particles is theoretically possible.
In a hitherto known method, gas components contained in a fluid medium are introduced into a container through a polymer membrane, and the concentration of the gas components are calculated based on changes in the thermal conductivity in the container (Japanese Laid-open Patent Publication (Kokai) No. H03-176640). This method has been employed to monitor the dissolved nitrogen concentration of a cleaning liquid.
Alternatively, Irradiation of ultrasonic waves onto sample water containing nitrogen gas generates hydrogen radicals derived from water molecules. The hydrogen radicals react with the nitrogen gas, and nitrogen compounds (NOx—, NH4+) are produced. A method based on this mechanism has been proposed for calculating the dissolved nitrogen concentration of a sample (Japanese Laid-open Patent Publication (Kokai) No. 2003-131308). In this method, after radical treatment is performed for sample water by means of ultrasonic wave irradiation, the ion amount derived from nitrogen atoms is measured by a resistivity meter, and the dissolved nitrogen concentration of the sample is calculated based on the ion amount.